A novel wavelength assignment method for a bidirectional two-fibre WDM network of star topology is proposed. It is shown that the minimal number of wavelengths required for an n-node star network is n (when n is odd) or n 1 (when n is even). A simple and explicit recursive formula, which completes wavelength assignment quickly with a time complexity of O(n2 log2(n)), is given. Simulation results show that the present method can give an optimal wavelength assignment much faster than conventional methods for a star network.

With a perfectly matched layer boundary treatment, a semivectorial finite-difference method is used to calculate the eigenmodes of a single-mode (SM) or multimode (MM) bent rib waveguide. A detailed analysis is given for the dependence of the bending losses (including the pure bending loss and the transition loss) on geometrical parameters of the bent rib waveguide such as the rib width, the rib height, and the bending radius. The characteristics of the higher-order modes are analyzed. It is shown that the bending loss of the fundamental mode can be reduced effectively by increasing the width and height of the rib. For an integrated device, undesired effects due to the higher-order modes of a MM bent waveguide can be removed by appropriate choice of the geometrical parameters. An appropriately designed MM bent waveguide is used to reduce effectively the bending loss of the fundamental mode, and a low-loss SM propagation in a MM bent waveguide is realized when the bending losses of the higher-order modes are large enough.

Silica waveguides with diameters larger than the wavelength of transmitted light are widely used in optical communications, sensors and other applications1-3. Minimizing the width of the waveguides is desirable for photonic device applications, but the fabrication of low-loss optical waveguides with subwavelength diameters remains challenging because of strict requirements on surface roughness and diameter uniformity4-7. Here we report the fabrication of subwavelength-diameter silica 'wires' for use as low-loss optical waveguides within the visible to near-infrared spectral range. We use a two-step drawing process to fabricate long free-standing silicawires with diameters down to 50nm that show surface smoothness at the atomic level together with uniformity of diameter. Light can be launched into these wires by optical evanescent coupling. The wires allow single-mode operation, and have an optical loss of less than 0.1dBmm21. We believe that these wires provide promising building blocks for future microphotonic devices with subwavelength-width structures.

A new method which combines the three-dimensional (3-D) beam propagation method with the overlap integral method is presented for the design of directional couplers. A backward beam propagation is calculated for the output region to reduce the computation time greatly. The design method is illustrated with buried silica-on-silicon waveguide couplers. The design results obtained with the 3-D model are compared with the results obtained with the two-dimensional model and it is shown that the 3-D model should be used. The design results are verified by both 3-D simulations and the experiment. The new design method is shown to be effective and accurate.

A numerical method based on the finite-difference time-domain (FDTD) scheme for computing defect modes in two-dimensional photonic crystals (with dielectric or metallic inclusions) is presented. Compared to the FDTD transmission spectra method, the present method reduces the computation domain significantly. By means of it one can find as many defect modes as possible, including those that are inactive to the incident plane wave in the FDTD transmission spectra method. The calculated eigenfrequencies and field patterns for a defect in a square array of dielectric rods are consistent with those obtained by the plane wave expansion method. Modes for a defect in a square array of copper rods are also studied, and the calculated eigenfrequencies are in a very good agreement with experimental results.